Having finished BBS I have been having some thoughts about what happened to Vanitas. It's all a little unclear. He disappeared after Ven shattered the x blade, so it could be that he's dead (ignoring kh3 Vani, because he's time traveling) but it's never that simple.
Here's a possible idea that I thought up: We're told that Sora was able to save Ven, by providing the piece that was missing. The piece that was missing was Vani.
Vani, from what I could tell, was motivated in BBS by the idea of re-fusing with Ven and being whole again (with extra x blade stuff either being a bonus or a Nort manipulation thing)
So. By that logic, we have a being who wants to be whole, but was rejected/couldn't because the hole was fixed. Something we don't know, though, is if there was any damage or consequences to Sora for repairing Ven's heart with his own (twice!).
If there is a missing piece there for Sora, that piece would be Vanitas shaped (carving a piece of that shape out to give to Ven). And Sora, who immediately accepted Ven with no questions asked, probably would have accepted Vani too.
Maybe Vani fused with Sora as a replacement for Ven, and that's where he went? And I mean a full fusion, like into one being, and not whatever the heck he and Roxas had.
It would explain some things like anti/rage form, and how he can wield two keyblades even after he has all of his passengers removed (because he would have void gear and kingdom key at that point)
I dunno if it works with the idea that Vani is an aspect of one of the Willful Darknesses from union cross though.
